Limitations:
As mentioned on the main page, Sigma Chess 6.1 Lite is freeware. Although it's a complete, full strength chess program and not just a demo, certain high end features of the commercial Sigma Chess 6.1 Pro version are either limited or disabled:
The new position and opening line filters are only available for collections with at most 100 games.
The new player rating history graph only shows the first 10 games.
Collections can contain a maximum of 1 games.
Changes to opening/position libraries cannot be saved.
Only the KQKR and KBNK endgame databases are included. The commercial Pro version contains several other 4-piece endings.
Automatic game annotation/analysis are disabled for collections.
Diagrams are not printed when printing collections/online chess books.
Diagrams are not included when exporting collections/online chess books to HTML
Transposition tables are limited to 10 MB, whereas the commercial version can handle transposition tables up to a size of 320 MB.
At most three windows can be opened simultaneously.
Monitor mode is not available.
UCI engines are limited to max 64 MB transposition/hash tables.
UCI engines cannot reduce the playing strength to a specific ELO setting.
UCI engines cannot access Nalimov tablebases.

What's New:
Now bundled with the new HIARCS 10 engine!
Sigma Chess 6.1 Pro can now host matches between chess engines (from the Engine Match... command in the Analyze menu). Is not supported under OS 9.
Fixed a bug in Sigma's transposition table handling.
Fixed a bug when sending FEN commands to UCI engines.
The Copy Position command now includes the move number and half moves since last capture or pawn move.
The PGN import now supports long algebraic notation too.
PGN import now allows various non-English letters.